Make the tests fast under the race detector (closes #120)

Deriving keys from passphrases with scrypt, slow on purpose, took most
of the test time under -race. secret.ScryptWorkFactor, when not zero,
replaces age's work factor when a passphrase encrypts; the tests of
internal/secret, internal/vault and internal/cli set it to 1 in
TestMain, and the program never sets it. Decryption reads the factor
from the encrypted data.

TestRemovalAsksWithoutHoldingLock and TestFailedCommandReleasesLock
wait at most 10 seconds for the in-memory lock all tests share, so they
no longer run in parallel with the tests that hold it.

The script/cibuild comment no longer says tests are skipped without
its memlock ulimit.

Model: opus-5-5

internal/cli/lock_test.go

@@ -235,9 +235,9 @@ func TestEncryptPipedIntoAdd(t *testing.T) {
 
 // TestFailedCommandReleasesLock checks that a command failing after it
 // took the state directory lock leaves the lock free for the next command.
+//
+//nolint:paralleltest // times commands against the in-memory lock all tests share
 func TestFailedCommandReleasesLock(t *testing.T) {
-	t.Parallel()
-
 	fs := afero.NewMemMapFs()
 	cli := NewCLIInstanceWithStateDir(fs, testStateDir)